1. Field of Invention
The present invention relates to a partial reflective laser output device. More particularly, the present invention relates to a device with which the laser output power can be controlled to meet eye safety standards and, at the same time, the reflected light is received by a photodiode for a feedback circuit to perform auto power control. The partial reflective laser output device of the present invention can avoid the problem of relative intensity noise and is most suitable for mass production.
2. Related Art
Referring to FIG. 1, the conventional surface emitting laser output device, such as a vertical cavity surface emitting laser (VCSEL) 10 comprises a P-type mirror 11 and an N-type mirror 13 enclosing an active area 12. Resonant light in the active area 12 forms laser light, wherein the up-going light 20 is output through the surface in a cone shape, while the down-going light is absorbed by a substrate 14. Only one surface of the VCSEL 10 emits light; therefore, auto power control on this system is not so easy as an edge emitting laser which emits light on both sides. Many suggestions have been proposed in the past, and most of them call for the monitoring with detectors of epitaxial chip structures grown from monoliths around the VCSEL 10. However, most of these suggestions, which do not allow a planar structure on the VCSEL 10, would cause the product yield and reliability to be adversely affected, as well as problems in the mass production thereof.
Therefore, if one wants to monitor the output light 20 intensity without changing the CSEL 10 structure, the output light 20 from the VCSEL 10 has to be partially directed to photodiode (PD). Most methods used on currently available products on the market are add a silicon (Si) PD under the VCSEL 10 and to encapsulate the devices in a transistor outline (TO) can. The above method for monitoring the output light intensity from the VCSEL 10 is the simplest one and can be achieved by coating a layer of reflective film on the lens of the TO can. On one hand, this makes the output light 20 from the VCSEL 10 comply with the Class I Eye Safety requirement of the range between 200 xcexcW and 400 xcexcW, and sufficient reflected light can be fed back to the PD to perform auto power control on the other. This method does not only keep a planar structure for the VCSEL 10 but still uses surface mount technology (SMT) in encapsulation. So it is very suitable for mass production processes.
With reference to FIG. 2A and taking a conventional flat window TO can encapsulation as an example, to monitor the output light 20 intensity from the TO can encapsulated VCSEL 10 the surface of the flat window 54 can be coated with a film so that reflected light 21 generated thereby can be absorbed by the PD 30 so as to monitor the output light 20 intensity. Under this encapsulation structure, however, an additional focusing lens, such as a ball lens 50, is needed in the exterior for coupling the VCSEL 10 output light 20 into a fiber 60. Thus, calibration of light paths becomes more difficult.
To decrease the complexity of coupling light, one can consider the encapsulation with a ball lens 50 TO can 52 structure, as shown in FIG. 2B, which integrates the ball lens 50 and the TO can 52 so that calibration of the coupling light paths between the VCSEL 10 output light 20 and the fiber 60 becomes much easier. Nonetheless, this results in another problem: since light is reflected by the spherical surface of the ball lens 50, the reflection angle is so large that only a small portion of reflected light reaches and is absorbed by the PD 30. So the light current detected by the PD 30 might not be enough for the enacting threshold required by the feedback circuit. Of course, the area of the PD 30 can be enlarged to receive sufficient light, yet this increases the cost.
Moreover, it is very difficult to coat a film on the ball lens 50 by mass production and, yet, the reflected light 21 would shine on the VCSEL 10 in the above-mentioned two encapsulation structures and causes the problem of relative intensity noise.
In view of the foregoing, it is an object of the present invention to provide a convenient device to monitor and adjust the output light intensity from a laser output device, e.g. a vertical cavity surface emitting laser (VCSEL), so as to meet the eye safety standard on one hand, and to avoid the problem of relative intensity noise due to light path overlapping on the other. Furthermore, this device requires a lower manufacturing cost.
Pursuant to the above object, a partial reflective laser output device of the present invention comprises a partial reflective unit mounted on a laser output device (such as a VCSEL), the partial reflective unit allowing the laser beam emitted from the laser output device to be partially reflected while the rest penetrating through. This device decreases the intensity of output laser light to comply with the eye safety standard on one hand; the reflected light is guided to be absorbed by a photodiode (PD) for performing auto power control on the laser output device on the other. In addition, by adjusting the tilting angle of the partial reflective unit so that it is not perpendicular to the output laser light path or making a proper curvature thereon, the-reflected light can have no destructive interference with the output light and even can be focused onto the PD so that there would be no relative intensity noise problem and the size of the PD can be made smaller to lower the cost.
Moreover, the present invention can be incorporated into a transistor outline (TO) can encapsulation or be mounted on a printed circuit board for practical uses in manufacture.